1. Field of the Invention
The present invention generally relates to an image drum cartridge and a developing unit.
2. Description of the Related Art
A conventional image forming apparatus such as a printer, a copying machine, and a facsimile machine utilizes an electrophotographic process. A surface of a photoconductive drum covered with a photoconductive insulating layer is charged uniformly. Then, the charged surface is exposed to a light image that represents print data. The light image selectively dissipates the charges on the surface of the photoconductive drum to form an electrostatic latent image on the surface of the photoconductive drum. Then, the electrostatic latent image is developed with a developer material (e.g., toner) containing a coloring agent into a toner image. The toner image is transferred onto print paper and subsequently fused under pressure and heat into a permanent image.
With a conventional image forming apparatus, when printing is performed in a low-duty mode intermittently in a low-temperature and low-humidity environment, the printed images are soiled. For example, if only a few lines of characters are printed on A4 size paper in an environment of 10° C. and 20% RH at a rate of one page per five minutes, the printed images becomes significantly soiled one to two hours after printing is initiated.
FIG. 2 illustrates a pertinent portion of a conventional developing unit.
Referring to FIG. 2, a developing roller 14 rotates in contact with a photoconductive drum 21 in a direction shown by arrow A, thereby applying toner to the photoconductive drum 21. A sponge roller 13 rotates in contact with the developing roller 14 in a direction shown by arrow B, thereby supplying toner to the developing roller 14. A developing blade 15 forms a thin layer of toner on the developing roller 14.
A good rule of thumb is that a printed image is soiled if the following relation is met,(Vdv+Et)−EOPC<C  Eq.(1)where EOPC is the surface potential of the photoconductive drum 21, Vdv is the voltage applied to the developing roller 14, Et is the surface potential of toner that forms a toner layer, and C is a constant unique to that electrophotographic printer.
This relation is due to the following fact. Actually, the toner layer on the developing roller 14 is not uniformly charged to a surface potential Et but with a certain distribution of charges. In other words, Et is an only average of the distributed charges. Thus, excessively charged toner particles may adhere to non-exposed areas on the photoconductive drum 21, causing soiling of printed images.
For example, in a low-temperature and low-humidity environment, if EOPC≈−800 V, Vdv=−220 V, and C=250, then Eq. (1) gives the following relation.(Vdv+Et)−EOPC=(−220+Et)−(−800)<250Therefore, Et<−330 V. This shows that values of Et<−330 V causes toner to be negatively charged excessively to give rise to soiling of printed images.
In a low-temperature and low-humidity environment, a charging roller 19 becomes dry to have a high electrical resistance, but receives the same constant voltage for charging the photoconductive drum 21. This causes a decrease in current injected into the photoconductive drum 21, so that the surface potential EOPC will increase correspondingly toward zero volts. The toner will also become dry to have a high electrical resistance, so that the toner is easily charged and the surface potential Et will be more negative.
When intermittent low-duty printing is performed, the toner that falls from a toner cartridge, not shown, becomes dense, especially on a portion P1 slightly upstream of the developing blade 15 with respect to the direction of rotation of the developing roller 14.
This phenomenon takes the form of a change in apparent density of toner with time. The apparent density is calculated by measuring the volume of the pile of toner a certain length of time after an amount of toner is introduced into a container. When toner is put in a container and left as it is, the toner is packed slowly to decrease in volume. As a result, the apparent density of the toner changes (usually increases). Table 1 lists apparent densities of pulverized toner and polymer toner.
TABLE 1time (min)00.512345610pulverized0.30610.30820.30950.31030.31030.31030.31030.31030.3103tonerpolymer0.41250.44090.46140.48960.50540.51100.51100.51380.5138toner
The apparent densities were calculated based on the volume of toner. A mixture of 30 grams pulverized toner and 30 grams polymer toner was agitated for 30 seconds and then put into a graduated cylinder of a 100-ml capacity. The toner in the graduated cylinder is left as it is for a certain time length without adding mechanical vibration. The apparent density is expressed in g/ml. As is clear from Table 1, the apparent density of pulverized toner does not change significantly but the apparent density of polymer toner increases with time until it substantially saturates after six minutes. The apparent density at this situation is referred to as saturation apparent density. The pulverized toner is manufactured by mechanically pulverizing and therefore the toner particles are in a variety of irregular shapes. Thus, nonuniform shapes of toner particles prevent the toner particles from being packed when the toner particles are left as they are. On the other hand, the particles of the polymer toner are uniform in size and generally spherical in shape, and therefore are apt to be packed when the particles are left as they are. In other words, the more spherical the toner particles are, the more easily the toner particles are packed.
Larger apparent densities cause a thicker toner layer on the developing roller 14 and a more negative surface potential Et.
As described above, performing low-duty printing in a low-temperature and low-humidity environment causes soiling of printed images and hence poor print quality.